Papers by Keyword: Solid Solution Hardening

Abstract: High density RF-DC plasma nitriding was carried out to harden the surface of martensitic stainless steels AISI-420. The nitriding temperature is hold at 673 K (or 400°C). Nitrogen (N₂) and mixture of nitrogen (N₂)-hydrogen (H₂) was utilized as a plasma atmosphere. Vickers micro-hardness testing was utilized to examine the change of surface hardness and the corresponding microstructure analysis was carried out by utilizing X-Ray Diffraction (XRD) analysis. Nitrogen concentration and diffusion length were characterized by using Auger Electron Spectroscopy (AES). Hydrogen addition to nitrogen plasma resulted significant increase of hardness, nitrogen concentration and diffusion length compared to pure nitrogen plasma. The significant peak shift observed in XRD pattern of specimen nitrided with N₂+H₂ plasma indicates new phase formation or phase transformation. Those phenomena related to hydrogen role in nitriding process.

Abstract: The proposed dislocation model describes the orientation dependence of the critical resolved shear stress (CRSS) and deformation mechanisms on the yield point in single crystals of austenitic stainless steel with nitrogen impurities. The model takes into account the following: the change of the interstitial atom position in the lattice from octahedral interstice to tetrahedral site owing to passage of a leading Shockley’s partial dislocation; the change in the separation width between two partial dislocation in external stress field; the relationship between the width of the extended dislocation and the elastic interaction of the extended dislocation with the impurity atoms.

Abstract: The single crystals were produced from industry zinc alloy containing 0.075wt.% of titanium and 0.15wt.% of copper. The Bridgman method was applied to obtain the single crystal. Three growth rates were used: 6mm/h, 10mm/h and 16mm/h. The common feature of all of the produced single crystals was homogeneous distribution of the second phase on a sections normal to the growth direction. The investigated single crystals were deformed in compression test with jump changes of the strain rate (from 10^-4s^-1 on 10^-3s^-1). From the obtained stress-strain curves the Critical Resolved Shear Stress (CRSS), hardening coefficient in the easy slip range (QA) and thermodynamical – activation volume (V*) and activation enthalpy (DH) were determined. The tests were done at the temperature range from 77K to 400K. Results showed anomaly on the CRSS=f(T) curve. The temperature of an extreme was 240K. The same effect was observed on V*=f(T) and DH=f(T) curves. The presented results reveal good correlation with crack phenomena that was observed during a bent of the zinc alloy sheets at the temperatures below 300K.

Abstract: Effect of nitrogen (N) addition on mechanical properties of Ti-Cr-Sn alloy was investigated in this study. Ti-7mol%Cr-3mol%Sn was selected and less than 0.5wt% of N were systematically added. The alloys were characterized by optical microscopy, X-ray diffraction analysis and tensile tests at room temperature. The apparent phase was β (bcc) phase, whereas the presence of precipitates was confirmed in 0.5wt%N-added alloy only which did not exhibit sufficient cold workability. The grain size was not largely affected by N addition being less than 0.5wt%. Tensile tests revealed that less than 0.5wt%N addition improves the strength which is due to the solution hardening by interstitial N atoms.

Abstract: The Kocks-Mecking method of analysis is applied to solid solutions of up to 2.6 at.% Zn to separate the contributions to the alloys’ strain hardening rate from dislocations storage, solute in solution, and twinning, for temperatures between -50 °C (273 K) and 200 °C (473 K). Athermal storage of dislocations seems to account for the largest share of the strain hardening rate for both the pure metal and the solid solutions at or below room temperature. Solute in solution does not increase the strain hardening rate over that of pure Mg, although it delays the onset of dynamic recovery, especially for the higher alloys, presumably due to short range order. Twinning remains a very important deformation mechanism for the pure metal and the dilute alloys up to 200 °C.

Abstract: Surface hardening treatment of titanium materials in use of CO gas was studied including investigation of post heat treatment under vacuum. C.P. titanium, α+β type SP-700 alloy with Ti-4.5%Al-3%V-2%Mo-2%Fe and β type alloy with Ti-15%Mo-5%Zr-3%Al were used. Surface hardening was conducted by heating these materials at 1073K for 21.6ks in Ar-5%CO gas. Subsequently, specimens subjected to surface hardening were heated at 1073k for various time periods under vacuum. While the maximum surface hardness value was the largest in C.P. titanium and the least in SP-700 alloy, hardening layer thickness was the thickest in β type alloy and the thinnest in C.P. titanium. Surface hardening in C.P. titanium was brought about by solid solution hardening due to oxygen and carbon. Enrichment of these elements in the surface layer of both titanium alloys caused continuous variations of the microstructure such as β to α+β, or their volume fractions in the surface hardening layer. Post heat treatment at 1073K increased the maximum surface hardness and hardening layer thickness with an extension of the heating time in C.P. titanium, but the surface maximum hardness decreased continuously in β type titanium alloy. Post heat treatment could remove the thin oxide layer formed by surface hardening treatment.

Abstract: High strength Al-8Fe-2Mo-2V-1Zr (wt.%) alloys fabricated by a melt spinning and a hot extrusion process were produced to correlate the microstructure and mechanical property. Melt spun ribbon prepared by single roll melt spinner showed a cellular structure with an average size of 10nm and Al-Fe based intermetallic dispersoid of less than 10nm in particle size. The melt spun ribbon obtained was then pulverized to make a powder shape followed by hot extrusion at 648K, 673K, 723K and 773K in extrusion ratio of 5 to 1, respectively. Equiaxed grain structure containing Al-Fe based intermetallic phase was observed in all extruded specimens. According to increasing extrusion temperature, the grain size increased and particle size of intermetallic dispersoid. The lattice parameter increased from 0.4051nm to 0.4059 nm with increasing extrusion temperature from 648K to 773K, those values were larger than that obtained in pure Al (0.4049nm). Yield strength of the specimen extruded at 648K measured to 956MPa at room temperature, 501MPa at 573K and 83MPa at 773K, respectively. With increasing extrusion temperature yield strength decreased significantly at room temperature and even in the intermediate temperature range, while no noticeable difference in yield strength was observed at 773K.

Abstract: Solid solution hardening effects of Ir was investigated to develop high temperature materials at 2223 K. Pt, Rh, Hf, and Zr were chosen as second elements because their solubility into Ir at 2223 K is over 2at% and the melting temperatures of Ir solid solution are above 2273 K. Compressive strength of Ir solid solution at 2223K were investigated. Solid solution hardening effect of Ir is discussed in terms of lattice parameter change and solubility,